Catalytic cracking is a well-know process that is being used in many refineries. In catalytic cracking a hydrocarbon feedstock is fed to a riser reactor into which also a cracking catalyst is fed. During the residence time in the riser reactor the hydrocarbon feedstock is being cracked into lighter products. Since the cracking reaction occurs at high temperatures the riser reactor is usually provided with insulation. Such insulation may be provided at the outside of the steel outer wall of the riser reactor. It is also possible to provide for a refractory lining inside the reactor which lining protects the steel outer wall from the erosive effect of the reaction mixture. At the same time it provides a layer of insulation to keep the outer wall cool. At cracking also some coke is being formed that deposits onto the cracking catalyst to yield spent catalyst. At the top of the riser reactor the product stream is separated from the spent catalyst, and the spent catalyst is then regenerated by burning off the coke using a regenerating gas. The regenerated catalyst is subsequently recycled to the riser reactor. The heat for the catalytic cracking reaction is supplied by the regenerated catalyst. The product stream of the catalytic cracking process is separated into various fractions, such as C4−-alkanes and C4−-olefins, naphtha, distillate oils and cycle oils in a fractionation column.
In the riser reactor the average linear gas velocity may be in the range of 10 to 30 m/s and the average velocity of the catalyst particles may be up to 25 m/s. The catalyst particles will move substantially co-currently with the gaseous reaction mixture. As the cracking reaction takes place on the catalyst particles, it is highly desirable that there is a good contact between the catalyst particles and the gaseous reaction mixture. Therefore, it has been proposed in U.S. Pat. No. 3,353,925 to provide the riser reactor with venturi-shaped contact devices. These contact devices have the shape of an annulus. These devices are basically narrowed portions of a refractory lining that is present anyway in the riser reactor.
It has further been found that the catalyst particles tend to flow in a core-annular flow pattern. This means that there are areas of dense catalyst concentration in the periphery of the riser reactor whilst leaving a diluted catalyst area in its centre. This has been acknowledged in U.S. Pat. No. 5,851,380. Such a flow pattern leads to inhomogeneous distribution of catalyst particles and sub-optimal conversion of the hydrocarbon feedstock. To solve this disadvantage it was proposed to provide the riser reactor with annular contact devices which create a turbulence and thereby a more homogeneous distribution of the catalyst particles. These contact devices may be arranged in any suitable means, but the description in U.S. Pat. No. 5,851,380 specifically discloses a piece of refractory in the desired shape interposed within the refractory lining of the riser reactor.
From the disclosures in the prior art it is evident that the mixture of hydrocarbon feed and catalyst particles provide a highly erosive environment. Therefore, the known contact devices are executed in refractory material. However, the cracking environment is also very hot. Temperatures between 480 and 640° C. are common. In these hot erosive environments there is the risk that the contact devices change in shape due to erosion and/or temperature effects. It is therefore desirable to fix the contact devices in such a way that these effects are minimised. Although U.S. Pat. No. 5,851,380 indicates that the disclosed contact devices may be attached to the riser reactor in any known way, no specific teaching as to what way one should chose has been provided. The solution disclosed refers to the interposition of a piece of refractory material with the refractory lining that is anyway present in the riser reactor. It has now been found that a more secure way to connect contact devices is provided by the use of a metal structure that is connected to the outer wall of the riser reactor.